The significant polarization components of a medium produced by contact with an electric field are first order polarization (linear polarization), second order polarization (first nonlinear polarization), and third order polarization (second nonlinear polarization). On a molecular level this can be expressed by Equation 1: EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 ( 1)
where
P is the total induced polarization, PA0 E is the local electric field created by electromagnetic radiation, and PA0 .alpha., .beta., and .gamma. are the first, second, and third order polarizabilities, each of which is a function of molecular properties. PA0 P is the total induced polarization, PA0 E is the local electric field created by electromagnetic radiation, and PA0 .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) are the first, second, and third order polarization susceptibilities of the electromagnetic wave transmission medium. PA0 S is a flexible spacer, and PA0 M is a pendant mesogen which exhibits a second order nonlinear optical susceptibility .beta. of at least about 20.times.10.sup.-30 esu under stated conditions of measurement. In one form the mesogen satisfies the formula: EQU --X--Y--Z
.beta. and .gamma. are also referred to as first and second hyperpolarizabilities, respectively. The molecular level terms of Equation 1 are first order or linear polarization .alpha.E, second order or first nonlinear polarization .beta.E.sup.2, and third order or second nonlinear polarization .gamma.E.sup.3.
On a macromolecular level corresponding relationships can be expressed by Equation 2: EQU P=.chi..sup.(1) E+.chi..sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3( 2)
where
.chi..sup.(2) and .chi..sup.(3) are also referred to as the first and second nonlinear polarization susceptibilities, respectively, of the transmission medium. The macromolecular level terms of Equation 2 are first order or linear polarization .chi.E, second order or first nonlinear polarization .chi..sup.(2) E.sup.2, and third order or second nonlinear polarization .chi..sup.(3) E.sup.3.
Second order polarization (.chi..sup.(2) E.sup.2) has been suggested to be useful for a variety of purposes, including optical rectification (converting electromagnetic radiation input into a DC output), generating an electro-optical (Pockels) effect (using combined electromagnetic radiation and DC inputs to alter during their application the refractive index of the medium), phase alteration of electromagnetic radiation, and parametric effects, most notably frequency doubling, also referred to as second harmonic generation (SHG).
To achieve on a macromolecular level second order polarization (.chi..sup.(2) E.sup.2) of any significant magnitude, it is essential that the transmission medium exhibit second order (first nonlinear) polarization susceptibilities, .chi..sup.(2), greater than 10.sup.-9 electrostatic units (esu). To realize such values of .chi..sup.(2) it is necessary that the first hyperpolarizability .beta. be greater than 10.sup.-30 esu. For a molecule to exhibit values of .beta. greater than zero, it is necessary that the molecule be asymmetrical about its center--that is, noncentrosymmetric. Further, the molecule must be capable of oscillating (i.e., resonating) between an excited state and a ground state differing in polarity. It has been observed experimentally and explained by theory that large .beta. values are the result of large differences between ground and excited state dipole moments as well as large oscillator strengths (i.e., large charge transfer resonance efficiencies).
For .chi..sup.(2) to exhibit a usefully large value it is not only necessary that .beta. be large, but, in addition, the molecular dipoles must be aligned so as to lack inversion symmetry. The largest values of .chi..sup.(2) are realized when the molecular dipoles are arranged in polar alignment--e.g., the alignment obtained when molecular dipoles are allowed to align themselves in an electric field.
D. J. Williams, "Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities", Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703, postulates mathematically and experimentally corroborates achievement of second order polarization susceptibilities .chi..sup.(2) using organic molecular dipoles equalling and exceeding those of conventional inorganic noncentrosymmetric dipole crystals, such a lithium niobate and potassium dihydrogen phosphate. To obtain the polar alignment of the organic molecular dipoles necessary to large values of .chi..sup.(2) Williams dispersed small amounts of the organic molecular dipoles as guest molecules in host liquid crystalline polymers. Upon heating the host polymers above their glass transition temperatures, poling in an externally applied electric field to produce the desired polar alignment of the molecular dipoles, and then cooling with the field applied, organic films with the measured levels of .chi..sup.(2) were obtained.
Zyss "Nonlinear Organic Materials for Integrated Optics", Journal of Molecular Electronics, Vol. 1, pp. 25-45, 1985, though generally cumulative with Williams, provides a review of passive linear light guide construction techniques and elaborates on LB film construction techniques including radiation patterning, showing in FIG. 8 an LB film construction converted into a linear polymer.
Recently attempts have been reported to prepare linear polymers containing pendant groups capable of acting as molecular dipoles for enhancing second order polarization effects. These attempts are illustrated by the following papers, all published in SPIE, Vol. 682, Molecular and Polymeric Optoelectronic Materials: Fudamentals and Applications (1986):
Le Barny, Ravaux, Dubois, Parneix, Njeumo, Legarnd, and Levelut, "Some New Side-Chain Liquid Crystalline Polymers for Non-Linear Optics", pp. 56-64, discloses unsuccessful attempts to obtain liquid crystal properties in vinyl addition copolymers containing aminostilbene pendant groups in concentrations of 2.6 percent.
Griffin, Bhatti, and Hung, "Synthesis of Sidechain Liquid Crystal Polymers for Nonlinear Optics", pp. 65-69, reports polyester copolymers containing stilbene molecular dipoles linked to the polymer backbone through an oxy electron donating moiety.
DeMartino et al U.S. Pat. No. 4,694,066 discloses a thermotropic liquid crystalline polymer which is characterized by a recurring monomeric unit of the formula: ##STR1## where P is a polymer main chain unit,
where EQU X is --NR--or --S--; EQU Y can take various stilbenoid forms; and EQU Z is an electron donating or withdrawing group,
the latter including nitro, haloalkyl, acyl, alkanoyloxy, alkoxysulfonyl, and the like.
S. Matsumoto, K. Kubodera, T. Kurihara, and T. Kaino, "Nonlinear Optical Properties of an Azo Dye Attached Polymer", App. Phys. Lett., Vol. 51, No. 6, Jul. 1987, pp. 1 and 2, discloses the synthesis of copolymers of azo dye disubstituted acrylic monomer and methyl methacrylate.